Optical parametric circuit

ABSTRACT

An optical parametric circuit for separating an input signal light and a wavelength-converted light even when the wavelength difference is small is provided. The optical parametric circuit includes: a nonlinear Mach-Zehnder interferometer which includes a first optical path and a second optical path; optical dispersive mediums and second-order optical nonlinear mediums provided in the first and second optical paths; wherein optical dispersive medium and second-order optical nonlinear medium in the first optical path are placed in the reverse order of optical dispersive medium and second-order optical nonlinear medium in the second optical path.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical parametric circuitwhich outputs a wavelength-converted light or a phase-conjugated lightof an input signal light by using optical parametric effect of opticalnonlinear mediums, and which can amplify these lights.

[0003] 2. Description of the Related Art

[0004] In recent years, elements which can convert a wavelength of lightwithout converting the light into an electric signal have beendeveloped. There are following three kinds of elements as examples.These three kinds of elements are an element which performs four-wavemixing by a semiconductor optical amplifier or an optical fiber, anelement which performs cross-gain modulation by a semiconductor opticalamplifier, and an element which performs cross-phase modulation bysemiconductor optical amplifiers.

[0005] As for four-wave mixing by the semiconductor optical amplifier,as shown in FIG. 1A, a signal light and a pump light are applied to asemiconductor optical amplifier 91 which is the optical nonlinearmedium. Then, a wavelength-converted light (four-wave mixed light) isgenerated and is separated from the signal light and the pump light byan optical filter 92 and is output, in which the light frequency(2fp-fs) of the wavelength-converted light and the light frequency (fs)of the signal light are symmetric with respect to the light frequency(fp) of the pump light.

[0006] As for the cross-gain modulation by the semiconductor opticalamplifier, as shown in FIG. 1B, when a signal light of wavelength λs isapplied to a semiconductor optical amplifier 93 which is rendered undergain saturation condition by applying a pump light of wavelength λp,gain for the pump light of wavelength λp decreases if the intensity ofthe signal light is high. Thus, the pump light of wavelength λp isoutput such that code represented by the pump light is logicallyinverted with respect to code represented by the signal light. Then, thepump light is separated from the signal light of wavelength λs by anoptical filter 94 and is output as the wavelength-converted light.

[0007] As for cross-phase modulation by semiconductor opticalamplifiers, as shown in FIG. 1C, a pump light of wavelength λp isdivided into two lights by an optical coupler 95-1 which lights areapplied to two semiconductor optical amplifiers 96-1 and 96-2. Inaddition, a signal light of wavelength λs is applied to thesemiconductor optical amplifier 96-1 via an optical coupler 95-2 fromthe opposite direction, in which output lights from the twosemiconductor optical amplifiers 96-1 and 96-2 are combined by anoptical coupler 95-3. When the signal light is applied to thesemiconductor optical amplifier 96-1, the refractive index of thesemiconductor optical amplifier 96-1 is changed so that the phase of thepump light which passes through the semiconductor optical amplifier 96-1is changed. Thus, the phases of the pump lights output from the twosemiconductor optical amplifiers 96-1 and 96-2 become different. As aresult, when the lights are mixed by the optical coupler 95-3, phasevariation appears as intensity variation. Therefore, the pump light ofwavelength λp which has the same logic code as that of the signal lightof λs is output from the output edge of the optical coupler 95-3 as thewavelength-converted light.

[0008] Semiconductor devices used in the above-mentioned structures havea limitation of response speed. Thus, it is technically difficult andcosts very much to process high speed signal higher than 40 Gbit/s.

[0009] In order to solve these problems, a wavelength conversiontechnique by using optical parametric process in a second-order opticalnonlinear medium is proposed. The response speed of the second-orderoptical nonlinear medium is high such that wavelength conversion ofultrahigh-speed optical signal faster than 100 Gbit/s is possible. Thewavelength conversion by using optical parametric process can beperformed by a third-order optical nonlinear medium. However, it isknown that, generally, the nonlinear coefficient of the second-orderoptical nonlinear medium is larger than that of the third-order opticalnonlinear medium so that the second-order optical nonlinear medium cangenerate the wavelength-converted light efficiently by using shortlength crystal (reference: M. H. cho, et al., IEEE photonics technologyletters, VOL. 11, pp. 653, 1999).

[0010] As for the wavelength conversion by using optical parametricprocess, conversion efficiency becomes higher when a traveling-wave typedevice is used and it is desirable that the wavelength conversion isperformed in a configuration configured such that an input signal lightand a wavelength-converted light are output in the same direction.However, according to this configuration, there is a problem in that,when wavelength difference between the input signal light and thewavelength-converted light is small, it becomes practically impossibleto separate the input signal light and the wavelength-converted light atthe output side.

SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide an opticalparametric circuit which realizes traveling-wave type parametricwavelength conversion of high efficiency and which can separate theinput signal light and the wavelength-converted light (orphase-conjugated light) even when the wavelength difference is small.

[0012] It is another object of the present invention to provide anoptical parametric circuit which can generate a phase-conjugated lighthaving no wavelength shift from the input signal light and which cancompletely separate the input signal light and the phase-conjugatedlight.

[0013] It is still another object of the present invention to provide anoptical parametric circuit which can output an amplifiedwavelength-converted light (or phase-conjugated light) from the inputsignal light.

[0014] The above objects of the present invention are achieved by anoptical parametric circuit comprising:

[0015] a first optical path which connects an output port of a firstoptical coupler and an input port of a second optical coupler;

[0016] a second optical path which connects an output port of the firstoptical coupler and an input port of the second optical coupler;

[0017] an optical dispersive medium and a second-order optical nonlinearmedium provided in each of the first optical path and the second opticalpath; and

[0018] wherein the optical dispersive medium and the second-orderoptical nonlinear medium in the first optical path are placed in thereverse order of the optical dispersive medium and the second-orderoptical nonlinear medium which are placed in the second optical path.

[0019] The above objects of the present invention are also achieved byan optical parametric circuit comprising:

[0020] an optical coupler;

[0021] a nonlinear loop mirror; and

[0022] wherein one output port of the optical coupler is connected toanother output port of the optical coupler via an optical dispersivemedium and a second-order optical nonlinear medium by the nonlinear loopmirror.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Other objects, features and advantages of the present inventionwill become more apparent from the following detailed description whenread in conjunction with the accompanying drawings, in which:

[0024] FIGS. 1A-1C show basic configurations of conventional wavelengthconversion elements;

[0025]FIG. 2 shows a basic configuration of the optical parametriccircuit of the present invention;

[0026]FIG. 3 shows a first embodiment of the optical parametric circuitof the present invention;

[0027]FIGS. 4A and 4B show a second embodiment of the optical parametriccircuit of the present invention;

[0028]FIG. 5 shows a third embodiment of the optical parametric circuitof the present invention;

[0029]FIGS. 6A and 6B show a fourth embodiment of the optical parametriccircuit of the present invention;

[0030]FIG. 7 is a figure for explaining effect of quasi-phase matchedLiNbO₃ waveguide;

[0031]FIG. 8 shows a fifth embodiment of the optical parametric circuitof the present invention;

[0032]FIG. 9 shows a sixth embodiment of the optical parametric circuitof the present invention;

[0033]FIGS. 10A and 10B show configuration examples of the opticaldispersive medium and the second-order optical nonlinear medium of thesixth embodiment.

[0034]FIGS. 11A and 11B are figures for explaining a problem inmodification 1 of the sixth embodiment;

[0035]FIGS. 12A and 12B are figures for explaining a method for using amodulation pattern;

[0036]FIGS. 13A and 13B show an example in which phase matchingcondition can be satisfied in eight pump light wavelength ranges byapplying three-fold modulation;

[0037]FIG. 14 shows quasi-phase matched LiNbO₃ waveguides 48, 49 inmodification 2 of the sixth embodiment;

[0038]FIG. 15 shows a seventh embodiment of the optical parametriccircuit of the present invention;

[0039]FIGS. 16A and 16B show an eighth embodiment of the opticalparametric circuit of the present invention;

[0040]FIG. 17 shows a ninth embodiment of the optical parametric circuitof the present invention;

[0041]FIGS. 18A and 18B show a tenth embodiment of the opticalparametric circuit of the present invention;

[0042]FIG. 19 shows an eleventh embodiment of the optical parametriccircuit of the present invention;

[0043]FIG. 20 shows a twelfth embodiment of the optical parametriccircuit of the present invention;

[0044]FIG. 21 shows a configuration example 1 of the optical dispersivemedium and the second-order optical nonlinear medium in the twelfthembodiment;

[0045]FIGS. 22A and 22B shows a configuration example 2 of the opticaldispersive medium and the second-order optical nonlinear medium in thetwelfth embodiment;

[0046]FIG. 23 shows a thirteenth embodiment of the optical parametriccircuit of the present invention; and

[0047]FIG. 24 shows a fourteenth embodiment of the optical parametriccircuit of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] In the following, the principle of the present invention will bedescribed.

[0049]FIG. 2 shows a basic configuration of the optical parametriccircuit of the present invention.

[0050] As shown in FIG. 2, the optical parametric circuit of the presentinvention is configured as a nonlinear Mach-Zehnder interferometer whichincludes two optical paths each of which optical paths includes anoptical dispersive medium and a second-order optical nonlinear medium.In this configuration, the placement order of the optical dispersivemedium and the second-order optical nonlinear medium in one optical pathof the two is the opposite of that of the other optical path.

[0051] A signal light and a pump light are mixed by a WDM coupler 10.The mixed light is input from an input port which is one of two inputports of an optical coupler 11 which has two input ports and two outputports so that the mixed light is divided into two optical paths. Themixed light which is divided and travels in one optical path is appliedto an optical dispersive medium 12 first, and, then, the mixed light isapplied to a second-order optical nonlinear medium 13. The mixed lightwhich is divided and which travels in the other optical path is appliedto a second-order optical nonlinear medium 14, and, then, the mixedlight is applied to an optical dispersive medium 15.Wavelength-converted lights generated by the second-order opticalnonlinear mediums 13, 14, and the signal light and the pump light whichpass through two optical paths are mixed by an optical coupler 16 whichhas two input ports and two output ports. As a result, the signal lightand the pump light are output from one of two output ports, and thewavelength-converted light is output from the other output port. Thatis, the optical parametric circuit can divide and output the signallight and the wavelength-converted light even when the wavelengthdifference between the signal light and the wavelength-converted lightis small or 0. The principle will be described in the following.

[0052] Assuming that electric fields of the signal light and the pumplight are Es, Ep, and that the angular frequencies are ω_(S), ω_(P)respectively, output electric field of the mixed lights which are outputto the two optical paths from the optical coupler can be represented bythe following equation (1). $\begin{matrix}{E_{in} = {\frac{1}{\sqrt{2}}\left\lbrack \left. \left. {Es} \middle| {{\exp \left( {i\quad \omega_{s}t} \right)} + {{{Ep}}{\exp \left( {i\quad \omega_{p}t} \right)}}} \right. \right\rbrack \right.}} & (1)\end{matrix}$

[0053] When the mixed light which is output to the one optical path isapplied to the optical dispersive medium 12, phase difference betweenthe signal light and the pump light is changed. The output electricfield can be represented by the following equation (2), $\begin{matrix}\begin{matrix}{E_{1}^{\prime} = {\frac{1}{\sqrt{2}}\left\lbrack {{{{Es}}{\exp \left( {{i\quad \omega_{s}t} - {i\quad {\beta \left( \omega_{s} \right)}L_{d}}} \right)}} +} \right.}} \\\left. {{{Ep}}{\exp \left( {{i\quad \omega_{p}t} - {i\quad {\beta \left( \omega_{p} \right)}L_{d}}} \right)}} \right\rbrack\end{matrix} & (2)\end{matrix}$

[0054] wherein β(ω_(S)) and β(ω_(P)) are propagation coefficients of theoptical dispersive medium 12 when optical angular frequencies are ω_(S)and ω_(P), L_(d) represents the length of the optical dispersive medium12. When the mixed light is applied to the second-order opticalnonlinear medium 13, the wavelength-converted light is generated. Theoutput electric field of the wavelength-converted light is representedby the following equation (3). $\begin{matrix}\begin{matrix}{E_{1}^{''} = \quad {\frac{1}{\sqrt{2}}\left\lbrack {{{{Es}}{\exp \left( {{i\quad \omega_{s}t} - {i\quad {\beta \left( \omega_{s} \right)}L_{d}}} \right)}} +} \right.}} \\{\quad {{{{Ep}}{\exp \left( {{i\quad \omega_{p}t} - {i\quad {\beta \left( \omega_{p} \right)}L_{d}}} \right)}} +}} \\\left. \quad {\sqrt{\eta_{PD}}{{Es}}{\exp \left( {{i\quad \omega_{p}t} - {i\quad \omega_{s}t} - {{i\left( {{\beta \left( \omega_{p} \right)} - {\beta \left( \omega_{s} \right)}} \right)}L_{d}}} \right)}} \right\rbrack\end{matrix} & (3)\end{matrix}$

[0055] η_(PD) which means wavelength conversion efficiency (ω_(P)→ω_(S))can be represented as the following equation (4), $\begin{matrix}\begin{matrix}{\eta_{PD} = {{\frac{1}{4}\left\lbrack {\frac{\omega_{s}\left( {\omega_{p} - \omega_{s}} \right)}{c^{3}ɛ_{0}^{3}n^{3}A}d^{2}} \right\rbrack}L_{n}^{2}\frac{P_{p}}{2}}} \\{= {\eta_{PD} \times \left( {L_{n}^{2}P_{P}} \right)}}\end{matrix} & (4)\end{matrix}$

[0056] wherein ω_(P)(rad/s) is the optical angular frequency of the pumplight, ω_(S)(rad/s) is the optical angular frequency of the signallight, ε₀(Fm⁻²) is the electric permeability of vacuum, c(m/s) is thelightwave velocity, n is the refractive index of the optical nonlinearmedium 13, L_(n)(m) is the effective length, A(m²) is the effectivecross-section area, d(m/V) is the nonlinear-dielectric constant,P_(P)(W) is the pump light power. η_(PD)′ is a conversion parameter.Each refractive index n of the second-order optical nonlinear medium 13for each wavelength is assumed to be the same. In addition, for the sakeof simplicity, it is assumed that energy of the signal light and thepump light is invariable. However, strictly speaking, the signal lightis amplified and the pump light is attenuated in the optical parametricprocess.

[0057] On the one hand, the mixed light which is output to the otheroptical path from the optical coupler 11 is applied to the second-orderoptical nonlinear medium 14 first so that wavelength-converted light isgenerated. The output electric field of the wavelength-converted lightis represented by the following equation (5). $\begin{matrix}\begin{matrix}{E_{2}^{\prime} = \quad {\frac{1}{\sqrt{2}}\left\lbrack {{{{Es}}{\exp \left( {i\quad \omega_{s}t} \right)}} + {{{Ep}}{\exp \left( {i\quad \omega_{p}t} \right)}} +} \right.}} \\\left. \quad {\sqrt{\eta_{PD}}{{Es}}{\exp \left( {{i\quad \omega_{p}t} - {i\quad \omega_{s}t}} \right)}} \right\rbrack\end{matrix} & (5)\end{matrix}$

[0058] Since this light is applied to the optical dispersive medium 15,the output electric field is represented by the following equation (6),$\begin{matrix}\begin{matrix}{E_{2}^{''} = \quad {\frac{1}{\sqrt{2}}\left\lbrack {{{{Es}}{\exp \left( {{i\quad \omega_{s}t} - {i\quad {\beta \left( \omega_{s} \right)}L_{d}}} \right)}} +} \right.}} \\{\quad {{{{Ep}}{\exp \left( {{i\quad \omega_{p}t} - {i\quad {\beta \left( \omega_{p} \right)}L_{d}}} \right)}} +}} \\\left. \quad {\sqrt{\eta_{PD}}{{Es}}{\exp \left( {{i\quad \omega_{p}t} - {i\quad \omega_{s}t} - {{i\left( {{\beta \left( \omega_{p} \right)} - {\beta \left( \omega_{s} \right)}} \right)}L_{d}}} \right)}} \right\rbrack\end{matrix} & (6)\end{matrix}$

[0059] wherein material parameters of the optical dispersive medium 15are the same as those of the optical dispersive medium 12.

[0060] Since the output light (represented by the equation (3)) from thesecond-order optical nonlinear medium 13 and the output light(represented by the equation (6)) from the optical dispersive medium 15are mixed at the optical coupler 16, output electric fields of theoutput port 1 and the output port 2 are represented by followingequations (7) and (8) respectively. $\begin{matrix}\begin{matrix}{E_{1,{OUT}} = \quad {{{{Es}}{\exp \left( {{i\quad \omega_{s}t} - {i\quad {\beta \left( \omega_{s} \right)}L_{d}}} \right)}} +}} \\{\quad {{{{Ep}}{\exp \left( {{i\quad \omega_{p}t} - {i\quad {\beta \left( \omega_{p} \right)}L_{d}}} \right)}} +}} \\{\quad {\sqrt{\frac{\eta_{PD}}{2\quad}}{{{Es}}\left\lbrack {\exp\left( {{i\quad \omega_{p}t} - {i\quad \omega_{s}t} - {i\left( {{\beta \left( \omega_{p} \right)} -} \right.}} \right.} \right.}}} \\{\left. {\left. \quad {\beta \left( \omega_{s} \right)} \right)L_{d}} \right) + {\exp\left( {{i\quad \omega_{p}t} - {i\quad \omega_{s}t} - {i\quad {\beta \left( {\omega_{p} - \omega_{s}} \right)}L_{d}}} \right\rbrack}}\end{matrix} & (7) \\\begin{matrix}{E_{2,{OUT}} = \quad {\sqrt{\frac{\eta_{PD}}{2\quad}}{{{Es}}\left\lbrack {{\exp \left( {{i\quad \omega_{p}t} - {i\quad \omega_{s}t} - {{i\left( {{\beta \left( \omega_{p} \right)} - {\beta \left( \omega_{s} \right)}} \right)}L_{d}}} \right)} +} \right.}}} \\{\quad {\exp\left( {{i\quad \omega_{p}t} - {i\quad \omega_{s}t} - {i\quad {\beta \left( {\omega_{p} - \omega_{s}} \right)}L_{d}}} \right\rbrack}}\end{matrix} & (8)\end{matrix}$

[0061] That is, the signal light and the pump light are output from theoutput port 1. The wavelength-converted light is output from both of theoutput port 1 and the output port 2. The power of thewavelength-converted light output from each of the output ports isrepresented by following equations (9) and (10).

P ₁=η_(PD) |Es| ²[1+cos {(β(ω_(P))−β(ω_(S))−β(ω_(P)−ω_(S)))L _(d)}]  (9)

P ₂=η_(PD) |Es| ²[1−cos {(β(ω_(P))−β(ω_(S))−β(ω_(P)−ω_(S)))L_(d)}]  (10)

[0062] As is understood from these equations, the output power of thewavelength-converted light varies according to phase shift provided bythe optical dispersive mediums 12 and 15. For example, in the opticaldispersive mediums 12 and 15, when a condition indicated by thefollowing equation (11) is satisfied

ΔβL _(d)=(β(ω_(P))−β(ω_(S))−β(ω_(P−ω) _(S)))L _(d)=(2n−1)π  (11)

[0063] wherein n is an integer, 100% of the wavelength converted lightis output from the output port 2 which is different from the output port1 from which the signal light and the pump light are output. Thus, thewavelength-converted light can be completely separated from the inputsignal light.

[0064] This is also true even when the wavelength of thewavelength-converted light is the same as that of the input signallight, wherein the wavelength-converted light of this case becomes aphase-conjugated light of the input signal light which has the samewavelength as that of the input signal light.

[0065] When η_(PD)>>1 in the equations (3) and (6), thewavelength-converted light (phase-conjugated light) is amplified withrespect to the input signal light. Thus, the optical parametric circuitfunctions as an optical parametric amplifier.

[0066] Although the above description is based on the configuration inwhich the signal light and the pump light are mixed and applied to oneinput port of the optical coupler 11 of the nonlinear Mach-Zehnderinterferometer, the optical parametric circuit can be also configuredsuch that each of the signal light and the pump light is applied to eachof the two input ports. Also in this case, the equation (11) representsthe condition for realizing complete separation between thewavelength-converted light (phase-conjugated light) and the input signallight. In this case, the pump light is output from the output port fromwhich the wavelength-converted light (phase-conjugated light) is output.

[0067] In the following, embodiments of the present invention will bedescribed.

[0068] (First Embodiment)

[0069]FIG. 3 shows a first embodiment of the optical parametric circuitof the present invention. In this embodiment, a configuration of theoptical parametric circuit will be described in which the wavelength ofthe signal light is 1550 nm which is suitable for optical transmission,the wavelength of the pump light is 775 nm which corresponds tosecond-order harmonic wave of the signal light, and phase-conjugatedlight which has the same wavelength 1550 nm as that of the input signallight is output.

[0070] In the figure, a semiconductor laser light source (LaserDiode:LD) 21 is driven by a power source 22 and oscillates at thewavelength 775 nm. The signal light and the pump light output from thesemiconductor laser light source 21 are mixed by a WDM coupler 23 andapplied to the nonlinear Mach-Zehnder interferometer 24. The nonlinearMach-Zehnder interferometer 24 includes two optical paths betweenoptical couplers 25 and 26 in which zinc-selenide 27 or 28 as theoptical dispersive medium and AANP crystal 29 or 30 are inserted in eachof the optical paths. The zinc-selenide 27 and the AANP crystal 29 formone optical path and the AANP crystal 30 and the zinc-selenide 28 formthe other optical path.

[0071] As for this configuration example, components are coupled viaspace wherein reflecting mirrors 31, 32 are provided in the two opticalpaths. In order that the effective optical path-lengths of the twooptical paths become identical, the lengths of the space part of the twooptical paths are set to be identical. In addition, the lengths of theAANP crystals 29 and 30 in the direction of light propagation are set tobe identical and the lengths of the zinc-selenides 27 and 28 in thedirection of light propagation are set to be identical.

[0072] Assuming that the length of the AANP crystals 29, 30 used as theoptical nonlinear medium is L_(n)=0.5 cm, the effective cross-sectionarea is A=500 μm², and the second-order nonlinear-dielectric constant isd=10⁻²²(MKS), the conversion parameter η_(PD)′ becomes

η_(PD)′=3.6×10⁻⁵(mW ⁻¹ cm ⁻²)   (12).

[0073] Assuming that the pump light power is P_(P)=1000 mW, wavelengthconversion efficiency η_(PD) becomes about 4.5×10⁻³(−23.5 dB).

[0074] The length L_(d) of the zinc-selenides 27, 28 can be calculatedas follows, wherein the length L_(d) is a key for separating the signallight and the generated phase-conjugated light completely at the outputstage of the optical parametric circuit. The refractive index of thezinc-selenide is n(λ_(p))=2.56 for pump light wavelength λ_(p)=775 nmand n(λ_(s))=2.47 for signal light wavelength λ_(s)=1550 nm. Thesevalues are substituted into the left-hand side of the equation (11) forobtaining the length L_(d) of the zinc-selenide such that the left-handside becomes equal to the right-hand side, wherein λ_(p)=2πc/ω_(P) andλ_(s)=2πc/ω_(S). According to the equation (11),

ΔβL _(d)=(β(ω_(P))−β(ω_(S))−β(ω_(P)−ω_(S)))L_(d)=2π(n(λ_(p))/λ_(p)2n(λ_(s))/λ_(s))L_(d)=2π(2.56/(775×10⁻⁹)−2×2.47/(1550×10⁻⁹))L _(d)=(2n−1)π (n is aninteger)   (13),

[0075] thus, L_(d)=4.3 μm (n=1) is the condition for separating thesignal light and the phase-conjugated light completely. Therefore, byconfiguring the optical parametric circuit by using a zinc-selenide filmof 4.3 μm long as the optical dispersive medium, it becomes possiblethat the phase-conjugated light having the same wavelength as that ofthe signal light can be separated from the signal light and output fromthe output port 2.

[0076] In addition, in the above-mentioned configuration, when thesignal light and the pump light are applied to the two input ports ofthe optical coupler 25 respectively, the signal light is output from theoutput port 1, and the phase-conjugated light and the pump light areoutput from the output port 2. In this case, since the wavelengthdifference between the phase-conjugated light and the pump light islarge, these can be easily separated.

[0077] As the optical parametric crystal forming the optical nonlinearmedium, MMA polymer and DAN crystal which are the same organic materialas the AANP crystal, LiNbO₃, LiTaO₃, KTP(KLiOPO₄), KDP(KH₂PO₄), KNbO₃and the like which are nonorganic crystal materials, GaN, ZnSSe,GaAs/GaAlAs semiconductor, AgGaSe₂, and AgGaS can be use in addition tothe above-mentioned AANP crystal. Further, as the optical dispersivemedium, materials such as fused-silica, synthesized sapphire, artificialquartz, and ceramic glass which has small thermal expansion constant andthe like can be used in addition to the zinc-selenide.

[0078] (Second Embodiment)

[0079]FIG. 4A shows the second embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configurationexample which realizes “wavelength channel exchange” for exchanging thesignal light wavelengths of a channel 1 and a channel 2, wherein thesignal light includes two channels of wavelengths 1545 nm and 1555 nmand the wavelength of the pump light is 775 nm which corresponds tosecond-order harmonic wave of the signal light.

[0080] The basic configuration of this embodiment using the nonlinearMach-Zehnder interferometer is almost the same as that of the firstembodiment except that synthesized sapphires 33, 34 are used as theoptical dispersive medium and LiTaO₃ crystals 35, 36 are used as thesecond-order optical nonlinear medium in this second embodiment.

[0081] In addition, as shown in FIG. 4B, electrodes 37, 38 areevaporated onto the both surfaces of the LiTaO₃ crystal 36 which isinserted into a second optical path of the nonlinear Mach-Zehnderinterferometer 24 in order that the optical path-length can be adjustedby applying voltage from a power source 39. The reason for adjusting theoptical path-length is that there may be cases where output ports forthe signal light and pump light, and the wavelength-converted light(phase-conjugated light) are swapped due to difference between opticalpath-lengths of the two optical paths of the nonlinear Mach-Zehnderinterferometer 24.

[0082] In this embodiment, a photoreceiver 40 is connected to the outputport 1 from which the signal light and the pump light should be output.The pump light is selectively monitored via a band-pass filter in thephotoreceiver 40 and a control circuit 41 receives detected output ofthe photoreceiver 40 according to the amount of received pump light. Thecontrol circuit 41 controls the power source 39 such that the amount ofreceived pump light becomes maximum. Accordingly, thewavelength-converted light can be output from the output port 2. In thisway, by providing such a feedback system for adjusting the optical pathlength, a robust system can be realized against manufacturing error ofthe nonlinear Mach-Zehnder interferometer 24.

[0083] Assuming that the length of the LiTaO₃ crystals 35, 36 used asthe optical nonlinear medium is L_(n)=0.5 cm, the effectivecross-section area is A=500 μm², and the second-ordernonlinear-dielectric constant is d=2.5×10³¹ ²²(MKS), the conversionparameter η_(PD)′ becomes

η_(PD)=4.7×10⁻⁵(mW ⁻¹cm⁻²)   (14).

[0084] Assuming that the pump light power is P_(P)=1000 mW, wavelengthconversion efficiency η_(PD) becomes about 5.125×10⁻³(−22.9 dB).

[0085] The length L_(d) of the synthesized sapphires 33, 34 can becalculated as follows, wherein the length L_(d) is a key for separatingthe signal light and the generated wavelength-converted light completelyat the output stage of the optical parametric circuit. The refractiveindex of the synthesized sapphire is n(λ_(p))=1.762 for pump lightwavelength λ_(p)=775 nm and n(λ_(s))=1.746 for signal light wavelengthλ_(s)=1550 nm. These values are substituted into the left-hand side ofthe equation (11) for obtaining the length L_(d) of the synthesizedsapphire such that the left-hand side becomes equal to the right-handside. According to the equation (11),

ΔβL _(d)=(β(ω_(P))−β(ω_(S))−β(ω_(P)−ω_(S)))L_(d)=2π(n(λ_(p))/λ_(p)−2n(λ_(s))/λ_(s))L_(d)=2π(1.762/(775×10⁻⁹)−2×1.746/(1550×10⁻⁹))L _(d)=(2n−1)π (n is aninteger)   (15),

[0086] thus, L_(d)=24.2 μm (n=1) is the condition for separating thesignal light and the wavelength-converted light completely. Therefore,by configuring the optical parametric circuit by using synthesizedsapphires of 24.2 μm long as the optical dispersive medium, it becomespossible that wavelength-converted lights, in which wavelengths of thesignal light of the channel 1 of 1545 nm and the channel 2 of 1555 nmare exchanged, are separated from the input signal light completely andoutput from the output port 2.

[0087] In addition, in the above-mentioned configuration, when thesignal light and the pump light are applied to the two input ports ofthe optical coupler 25 respectively, the signal light is output from theoutput port 1, and the wavelength-converted light and the pump light areoutput from the output port 2. Therefore, in this case, thephotoreceiver 40 which monitors the pump light is connected to theoutput port 2 via an optical coupler. Also in this case, since thewavelength difference between the wavelength-converted light and thepump light is large, these can be easily separated. The same goes forembodiments which will be described below.

[0088] As the optical parametric crystal forming the optical nonlinearmedium, LiNbO₃, KTP(KLiOPO₄), KDP(KH₂PO₄), KNbO₃ and the like which arenonorganic crystal materials same as the LiTaO₃ crystal, and AANPcrystal, MMA polymer and DAN crystal which are organic materials, GaN,ZnSSe, GaAs/GaAlAs semiconductor, AgGaSe₂, and AgGaS can be used inaddition to the above-mentioned LiTaO₃ crystal. Further, as the opticaldispersive medium, in addition to the synthesized sapphire, materialssuch as zinc-selenide, fused-silica, artificial quartz, and ceramicglass which has small thermal expansion constant and the like can beused.

[0089] (Third Embodiment)

[0090]FIG. 5 shows the third embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1550 nm which is suitable for opticaltransmission, the wavelength of the pump light is 775 nm whichcorresponds to second-order harmonic wave of the signal light, andphase-conjugated light which has the same wavelength 1550 nm as that ofthe input signal light is output.

[0091] The basic configuration of this embodiment using the nonlinearMach-Zehnder interferometer is almost the same as that of the firstembodiment except that fused-silicas 42, 43 are used as the opticaldispersive medium and GaAlAs semiconductor waveguides 44, 45 are used asthe second-order optical nonlinear medium in this second embodiment.Power sources 46, 47 are connected to the GaAlAs semiconductorwaveguides 44, 45 respectively wherein current of 50 mA is applied toeach GaAlAs semiconductor waveguide.

[0092] In addition, like the second embodiment, the optical parametriccircuit of the third embodiment includes the photoreceiver 40 monitoringthe pump light power and the control circuit 41 such that the powersource 47 obtains feedback from the control circuit 41 for compensatingfor the optical path-length error of the nonlinear Mach-Zehnderinterferometer 24. Accordingly, the phase-conjugated light can be outputfrom the output port 2 and a robust system can be realized againstmanufacturing error of the nonlinear Mach-Zehnder interferometer 24.

[0093] The GaAlAs semiconductor waveguides 44, 45 which are used as thesecond-order optical nonlinear medium have waveguide structure. By usingthe waveguide structure, the effect of light confinement of the pumplight is heightened so that the effective cross-section area is reduced,and conversion efficiency increases while same pump light power is used.

[0094] Assuming that the length of the GaAlAs semiconductor waveguide isL_(n)=0.2 cm, the effective cross-section area is A=50 μm², and thesecond-order nonlinear-dielectric constant is d=1.3×10⁻²¹(MKS), theconversion parameter η_(PD)′ becomes

η_(PD)′=6.26×10⁻²(mW ⁻¹ cm ⁻²)   (16)

[0095] according to the equation (4).

[0096] Thus, when the pump light power is P_(P)=100 mW, the wavelengthconversion efficiency η_(PD) becomes about 1.25×10⁻¹(−9.03 dB).Therefore, according to this embodiment, considerably higher conversionefficiency can be obtained while the pump light power which is smallerby an order of magnitude than that used in the second embodiment isused.

[0097] The length L_(d) of the fused-silicas 42, 43 can be calculated asfollows, wherein the length L_(d) is a key for separating the signallight and the generated phase-conjugated light completely at the outputstage of the optical parametric circuit. The refractive index of thefused-silica is n(λ_(p))=1.454 for pump light wavelength λ_(p)=775 nmand n(λ_(s))=1.444 for signal light wavelength λ_(s)=1550 nm. Therefore,according to the equation (11),

ΔβL _(d)=(β(ω_(P))−β(ω_(S))−β(ω_(P)−ω_(S)))L_(d)=2π(n(λ_(p))/λ_(p)−2n(λ_(s))/λ_(s))L_(d)=2π(1.454/(775×10⁻⁹)−2×1.444/(1550×10⁻⁹))L _(d)=(2n−1)π (n is aninteger)   (17),

[0098] thus, L_(d)=38.8 μm (n=1) is the condition for separating thesignal light and the phase-conjugated light completely. Therefore, byconfiguring the optical parametric circuit by using the fused-silica of38.8 μm long as the optical dispersive medium, it becomes possible thatthe phase-conjugated light having the same wavelength as that of thesignal light can be separated from the signal light and output from theoutput port 2.

[0099] As the optical waveguide used in this embodiment, in addition tothe above-mentioned GaAlAs semiconductor, materials of GaN familysemiconductor, ZnSSe family semiconductor, LiNbO₃, LiTaO₃, KTP(KLiOPO₄),KDP(KH₂PO₄), KNbO₃ and the like can be used. In addition, as the opticaldispersive medium, in addition to the above-mentioned fused-silica (38.8μm long), zinc-selenide (first embodiment: 4.3 μm long), synthesizedsapphire (second embodiment: 24.2 μm long), artificial quartz, andceramic glass which has small thermal expansion constant can be used.

[0100] (Fourth Embodiment)

[0101]FIG. 6 shows the fourth embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1549 nm, the wavelength of the pump light is 775nm which corresponds to second-order harmonic wave of the signal light,and wavelength-converted light of 1551 nm is output.

[0102] The basic configuration of this embodiment using the nonlinearMach-Zehnder interferometer is almost the same as that of the firstembodiment except that synthesized sapphires 33, 34 are used as theoptical dispersive medium and quasi-phase matched LiNbO₃ waveguides 48,49 are used as the second-order optical nonlinear medium in this fourthembodiment. As shown in FIG. 6B, the quasi-phase matched LiNbO₃waveguides 48, 49 are two LiNbO₃ waveguides which are formed such thatdipole-inversion regions 50 are formed on a LiNbO₃ substrate 53 atestablished intervals by applying voltage at the time of manufacturingand the two LiNbO₃ waveguides are formed by diffusing titanium (Ti) onthe substrate. By adopting this waveguide structure, the effect of lightconfinement of the pump light is heightened so that the effectivecross-section area is reduced, and phase match between the pump lightand the signal light is realized.

[0103] In the third embodiment, GaAlAs semiconductor waveguides 44, 45are used as the optical nonlinear medium. Since difference betweenrefractive indexes of the pump light and the signal light is very large,phase-mismatch is accumulated as shown in FIG. 7. Therefore, it isdifficult to realize phase matching. This problem also occurs when usingwaveguide of nonorganic crystal such as LiNbO₃ or waveguide of organiccrystal such as AANP. Thus, this problem is a general problem. Theperiodic dipole-inversion structure is generally used for solving thisproblem. The accumulation of phase-mismatch between the pump light andthe signal light is suppressed by inverting dipole direction of LiNbO₃at established cycle. Thus, it becomes possible to lengthen thewaveguide while satisfying the phase matching condition. As a result,conversion efficiency can be increased while the pump light power is thesame.

[0104] In addition, like the second embodiment, for compensating for theoptical path-length error of the nonlinear Mach-Zehnder interferometer24, the optical parametric circuit of the fourth embodiment includeselectrodes 37, 38 and the power source 39 which apply voltage to thesecond-order optical nonlinear medium, the photoreceiver 40 whichmonitors the pump light power and the control circuit 41 such that thepower source 39 obtains feedback from the control circuit 41.Accordingly, the wavelength-converted light can be output from theoutput port 2 and a robust system can be realized against manufacturingerror of the nonlinear Mach-Zehnder interferometer 24.

[0105] Assuming that the length of the quasi-phase matched LiNbO₃waveguides 48, 49 used as the optical nonlinear medium is L_(n)=6 cm,the effective cross-section area is A=50 μm², and the second-ordernonlinear-dielectric constant is d=5×10⁻²³(MKS), the conversionparameter η_(PD)′ becomes

η_(PD)′=9.2×10⁻⁵(mW ⁻¹ cm ⁻²)   (18)

[0106] according to the equation (4).

[0107] Thus, when the pump light power is P_(P)=1000 mW, the wavelengthconversion efficiency η_(PD) becomes about 1.66(2.2 dB). Thus, in thiscase, not only wavelength conversion but also amplification ofwavelength-converted light is possible.

[0108] The length L_(d) of the synthesized sapphires 33, 34, which is akey for separating the signal light and the generatedwavelength-converted light completely at the output stage of the opticalparametric circuit, is the same as that in the second embodiment. Thatis, by configuring the optical parametric circuit by using thesynthesized sapphire of 24.2 μm long as the optical dispersive medium,it becomes possible that the wavelength-converted light can be separatedfrom the signal light and output from the output port 2.

[0109] As the quasi-phase matched waveguide forming the opticalnonlinear medium, materials such as LiTaO₃, KTP(KLiOPO₄), KDP(KH₂PO₄)and KNbO₃, and, GaAlAs family semiconductor, GaN family semiconductorand ZnSSe family semiconductor can be used in addition to theabove-mentioned LiNbO₃. Further, in addition to the above-mentionedsynthesized sapphire, zinc-selenide, fused-silica, artificial quartz,and ceramic glass which has small thermal expansion constant can be usedas the optical dispersive medium.

[0110] (Fifth Embodiment)

[0111]FIG. 8 shows the fifth embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1550 nm, the wavelength of the pump light is 775nm which corresponds to second-order harmonic wave of the signal light,and, phase-conjugated light of 1550 nm which is the same as that of theinput signal light is output.

[0112] The basic configuration of this embodiment is almost the same asthat of the fourth embodiment. The difference is that, in thisembodiment, fused-silicas 42, 43 are used as the optical dispersivemedium, and, the fused-silicas 42, 43 and quasi-phase matched LiNbO₃waveguides 48, 49 used as the second-order optical nonlinear medium areformed monolithically by evaporate the fused-silica onto input edgesurface or output edge surface of the quasi-phase matched LiNbO₃waveguides 48, 49. The evaporation thickness of the fused-silicas 42, 43is 38.8 μm which is the same as the length of the fused-silica used inthe third embodiment as the optical dispersive medium. Accordingly, itbecomes possible to reduce cost by reducing number of optical circuitparts while the optical parametric circuit of this embodiment has thesame performance as the fourth embodiment, and the optical parametriccircuit of this embodiment becomes suitable for mass production.

[0113] As the optical dispersive medium used in this embodiment,synthesized sapphire film, silica film, dielectric film in whichdielectric mediums such as several glass materials of differentrefractive indexes are laminated.

[0114] (Sixth Embodiment)

[0115]FIG. 9 shows a sixth embodiment of the optical parametric circuitof the present invention. In this embodiment, a configuration of theoptical parametric circuit will be described in which the wavelength ofthe signal light is 1550 nm, the wavelength of the pump light is 775 nmwhich corresponds to second-order harmonic wave of the signal light, andphase-conjugated light which has the same wavelength as that of theinput signal light is output.

[0116] The basic configuration of this embodiment is almost the same asthat of the fourth embodiment except that, in this embodiment, phasemismatched LiNbO₃ waveguides 51, 52 as the optical dispersive medium andthe quasi-phase matched LiNbO₃ waveguides 48, 49 as the second-orderoptical nonlinear medium are formed monolithically on an LiNbO₃substrate 53. As was described in the fourth embodiment, difference ofpropagation coefficient between the pump light and the signal light in anormal LiNbO₃ waveguide is large. In this embodiment, thischaracteristic is used for realizing the optical dispersive medium,which is the phase mismatched LiNbO₃ waveguides 51, 52. Accordingly, theoptical dispersive medium and the optical nonlinear medium can beaccumulated. Thus, a coupling loss and the like can be decreasedcomparing with the fourth embodiment, and, it becomes possible to reducecost by reducing number of optical circuit parts so that the opticalparametric circuit of the present invention is suitable for massproduction.

[0117] In addition, like the second embodiment, for compensating for theoptical path-length error of the nonlinear Mach-Zehnder interferometer24, the optical parametric circuit of this embodiment includes the powersource 39 which apply voltage to the second-order optical nonlinearmedium, the photoreceiver 40 which monitors the pump light power and thecontrol circuit 41 such that the power source 39 obtains feedback fromthe control circuit 41. Accordingly, the wavelength-converted light canbe output from the output port 2 and a robust system can be realizedagainst manufacturing error of the nonlinear Mach-Zehnder interferometer24.

[0118] The wavelength conversion efficiency η_(PD) by using thequasi-phase matched LiNbO₃ waveguides 48, 49 as the second-order opticalnonlinear medium becomes about 1.66(2.2 dB) under the same condition asthe fourth embodiment. In this case, amplification of thephase-conjugated light is also possible.

[0119] The length L_(d) of the phase mismatched LiNbO₃ waveguides 51, 52can be calculated as follows, wherein the length L_(d) is a key forseparating the signal light and the generated phase-conjugated lightcompletely at the output stage of the optical parametric circuit. Therefractive index of the phase mismatched LiNbO₃ waveguides 51, 52 isn(λ_(p))=2.26 for pump light wavelength λ_(p)=775 nm and n(λ_(s))=2.22for signal light wavelength λ_(s)=1550 nm. Thus, according to theequation (11),

ΔβL _(d)=(β(ω_(P))−β(ω_(S))−β(ω_(P)−ω_(S)))L_(d)=2π(n(λ_(p))/λ_(p)−2n(λ_(s))/λ_(s))L_(d)=2π(2.26/(775×10⁻⁹)−2×2.22/(1550×10⁻⁹))L _(d)=(2n1)π (n is aninteger)   (19),

[0120] thus, L_(d)=9.7 μm (n=1) is the condition for separating thesignal light and the phase-conjugated light completely. Therefore, byconfiguring the optical parametric circuit by using the phase mismatchedLiNbO₃ waveguide of 9.7 μm long as the optical dispersive medium, itbecomes possible that the phase-conjugated light which has the samewavelength as the signal light can be separated from the signal lightcompletely and be output from the output port 2.

[0121] As optical waveguides used for this embodiment, materials ofLiTaO₃, KTP(KLiOPO₄), KDP(KH₂PO₄), KNbO₃ and the like can be used inaddition to the LiNbO₃.

[0122] (Modification 1 Of The Sixth Embodiment)

[0123] As shown in FIG. 6B, the quasi-phase matched LiNbO₃ waveguides48, 49 are formed on the LiNbO₃ substrate 53, on which LiNbO₃ substrate53 dipole-inversion regions 50 are placed at established intervalsbetween which intervals dipole-noninversion regions are formed. When thelength of the dipole-noninversion region of the quasi-phase matchedLiNbO₃ waveguides 48, 49 is set to be the same as the length L_(d) ofeach of the phase mismatched LiNbO₃ waveguides 51, 52, it becomespossible that one of the dipole-noninversion regions of the quasi-phasematched LiNbO₃ waveguides 48, 49 can be used as each of the phasemismatched LiNbO₃ waveguides 51, 52.

[0124]FIGS. 10A and 10B show configuration examples of the opticaldispersive medium and the second-order optical nonlinear medium in themodification 1 of the sixth embodiment. In the quasi-phase matchedLiNbO₃ waveguides 48, 49 shown in the figures, absolute values ofnonlinear-dielectric constant d (used in the equation (4)) for thedipole-inversion regions 50 and the dipole-noninversion regions 70 arethe same and the signs of the values are opposite. That is, the lengthsof the dipole-inversion regions 50 and the dipole-noninversion regions70 are the same wherein the length is one-half of dipole-inversioncycle.

[0125] As shown in FIG. 10A, in the quasi-phase matched LiNbO₃waveguides 48, 49, the dipole-inversion regions 50 and thedipole-noninversion regions 70 are formed alternately in which thelength of each region is 9.7 μm. Accordingly, the firstdipole-noninversion region 70 in the quasi-phase matched LiNbO₃waveguide 48 can be used as the phase mismatched LiNbO₃ waveguide 51 as9.7 μm long optical dispersive medium, and the last dipole-noninversionregion 70 in the quasi-phase matched LiNbO₃ waveguide 49 can be used asthe phase mismatched LiNbO₃ waveguide 52 as another 9.7 μm long opticaldispersive medium. That is, from outward appearances, there is nodifference between the optical dispersive medium and the opticalnonlinear medium.

[0126] In addition, as shown in FIG. 10B, the same effect can beobtained by using an LiNbO₃ substrate 53 on which the quasi-phasematched LiNbO₃ waveguides 48, 49 are formed by shifting one-half of thedipole-inversion cycle such that the positions of the dipole-inversionregion 50 and the dipole-noninversion region 70 between the quasi-phasematched LiNbO₃ waveguides 48 and 49 become the same in light travelingdirection.

[0127] (Modification 2 Of The Sixth Embodiment)

[0128] According to the modification 1 of the sixth embodiment, there isan advantage in that it becomes possible that one of thedipole-noninversion regions of the quasi-phase matched LiNbO₃ waveguides48, 49 can be used as each of the phase mismatched LiNbO₃ waveguides 51,52. However, there is a following problem for this case. That is, thereis only one wavelength range of the pump light of 780 nm range forheightening the conversion efficiency and for satisfying the phasematching condition, and the one wavelength range is very narrow. Thus,the pump light wavelength or converted wavelength of thewavelength-converted light can not be selected.

[0129] That is, as for a quasi-phase matched LiNbO₃ waveguide(dipole-inversion waveguide) on which waveguide parts havingnonlinear-dielectric constants d and −d are placed alternately as shownin FIG. 11A, only one band of the pump light for heightening theconversion efficiency appears and it is very narrow (about 1 nm) asshown in FIG. 11B.

[0130] For solving this problem, there is a method of modulating thedipole-inversion waveguide. By using this method, it becomes possiblethat the phase matching condition can be satisfied for a plurality ofpump light wavelength bands.

[0131] For example, a modulation pattern shown as (1) in FIG. 12A ismultiplied to a dipole-inversion waveguide in which waveguide partshaving nonlinear-dielectric constants d and −d are placed alternatelyshown as (2) in FIG. 12A. As a result, a dipole-inversion waveguideshown as (3) is obtained. By using this dipole-inversion waveguide, thenumber of the wavelength ranges of the pump light for satisfying thephase matching condition becomes two as shown in FIG. 12B. In this case,the pump light can be selected from two wavelengths. In the abovedescription, “multiply” means that, for example, when a part of thedipole-inversion waveguide which exists at +1 position of the modulationpattern is d, +1×d is calculated, and, when the part is −d, +1×(−d) iscalculated.

[0132] In this embodiment, an example is shown in which the phasematching condition can be satisfied in eight pump light wavelengthranges by performing three-fold modulation as shown in FIG. 13A. Themodulation cycles are 14 mm, 7 mm and 3.5 mm respectively. Accordingly,as shown in FIG. 13B, the phase matching wavelengths are formed at 0.8nm intervals centering on 775 nm which corresponds to the second-orderharmonic wave of 1550 nm, that is, the phase matching wavelengths are772.2, 773.0, 773.8, 774.6, 775.4, 776.2, 777.0 and 777.8 nm.

[0133] Also in this case, dipole-inversions of the quasi-phase matchedLiNbO₃ waveguides 48, 49 are opposite each other as shown in FIG. 14.

[0134] According to this configuration, by changing the pump lightwavelength, the wavelength of the wavelength-converted light can bechanged. For example, by selecting the pump light properly for inputsignal light of 1550 nm, the wavelength of the wavelength-convertedlight can be selected from a range of 1515 nm-1585 nm.

[0135] (Seventh Embodiment)

[0136]FIG. 15 shows a seventh embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1550 nm, the wavelength of the pump light is 775nm which corresponds to second-order harmonic wave of the signal light,and phase-conjugated light which has the same wavelength 1550 nm as thatof the input signal light is output.

[0137] The basic configuration of this embodiment is almost the same asthat of the sixth embodiment except that, in this embodiment, thenonlinear Mach-Zehnder interferometer 24 as a whole is configured byoptical waveguides on the LiNbO₃ substrate 53. That is, optical couplers25, 26, the phase mismatched LiNbO₃ waveguides 51, 52 used as theoptical dispersive medium and the quasi-phase matched LiNbO₃ waveguides48, 49 used as the second-order optical nonlinear medium are formedmonolithically. Accordingly, while this circuit has the same performanceas that of the sixth embodiment, it becomes possible to reduce cost byreducing number of optical circuit parts so that the optical parametriccircuit is suitable for mass production.

[0138] The wavelength conversion efficiency η_(PD) by using thequasi-phase matched LiNbO₃ waveguides 48, 49 as the second-order opticalnonlinear medium becomes about 1.66(2.2 dB) under the same condition asthe fourth embodiment. In this case, amplification of thephase-conjugated light is also possible.

[0139] The length L_(d) of the phase mismatched LiNbO₃ waveguides 51, 52becomes L_(d)=9.7 μm under the same condition as the sixth embodiment.That is, by configuring the optical parametric circuit by using thephase mismatched LiNbO₃ waveguide of 9.7 μm long as the opticaldispersive medium, it becomes possible that the phase-conjugated lightwhich has the same wavelength as the signal light can be separated fromthe signal light completely and be output from the output port 2. Inaddition, the phase mismatched LiNbO₃ waveguides 51, 52 and thequasi-phase matched LiNbO₃ waveguides 48, 49 can be configured as shownin FIG. 10A, which holds true for following each embodiment.

[0140] As optical waveguides used for this embodiment, materials ofLiTaO₃, KTP(KLiOPO₄), KDP(KH₂PO₄), KNbO₃ and the like can be used inaddition to the LiNbO₃.

[0141] (Eighth Embodiment)

[0142]FIG. 16A shows an eighth embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1550 nm, the wavelength of the pump light is 775nm which corresponds to second-order harmonic wave of the signal light,and phase-conjugated light which has the same wavelength 1550 nm as thatof the input signal light is output.

[0143] The basic configuration of this embodiment is almost the same asthat of the seventh embodiment except that, in this embodiment, thenonlinear Mach-Zehnder interferometer 24 as a whole is configured byoptical waveguides on a silica substrate 60. That is, optical couplers25, 26, the silica optical waveguides 61-64 used as the opticaldispersive medium and the quasi-phase matched GaAs/AlGaAs waveguides 65,66 used as the second-order optical nonlinear medium are formedmonolithically. The silica optical waveguides 61-64 are used as theoptical dispersive medium because difference of propagation coefficientsbetween the signal light and the pump light is large. From outwardappearances, four optical dispersive mediums are placed at both ends ofinput and output of the optical nonlinear mediums in this embodiment.Each of the difference of the lengths of the silica optical waveguides61, 62 which are in the input side and the difference of the lengths ofthe silica optical waveguides 63, 64 which are in the output sidefunctions as the optical dispersive medium for separating the signallight and the phase-conjugated light (wavelength-converted light).

[0144] As shown in FIG. 16B, the quasi-phase matched GaAs/AlGaAswaveguides 65, 66 are configured such that dipole-inversion regions inwhich placement of atoms of Ga and atoms of As are inverted as shown inFIG. 16B are placed cyclically in the direction of the length of thewaveguides. A manufacturing process for the waveguides is disclosed, forexample, in Japanese Journal of Applied Physics, Shinji Koh et al.,“GaAs/Ge/GaAs Sublattice Reversal Epitaxy on GaAs (100) and (111)Substrates for nonlinear Optical Devices”, Jpn. J. Appl. Phys. Vol. 38,L508-L511, 1999.

[0145] According to this waveguide structure, the effectivecross-section area is reduced so that the effect of light confinement ofthe pump light is heightened, and, phase matching is realized betweenthe pump light and the signal light.

[0146] Assuming that the length of the quasi-phase matched GaAs/AlGaAswaveguides 65, 66 is L_(n)=1 mm, the effective cross-section area isA=12.5 μm², and the second-order nonlinear-dielectric constant isd=1×10⁻²¹(MKS) and the refractive index is 3.5, the conversion parameterη_(PD)′ becomes

η_(PD)′=1.6×10⁻⁴(mW ⁻¹ cm ⁻²)   (20).

[0147] Thus, when the pump light power is P_(P)=500 mW, the wavelengthconversion efficiency η_(PD) becomes about 2.7(1.4 dB). In this case,not only wavelength conversion but also amplification ofphase-conjugated light is possible.

[0148] The difference L_(d) of the lengths of the silica opticalwaveguides 61 and 62 (63 and 64), which is a key for separating thesignal light and the generated phase-conjugated light completely at theoutput stage of the optical parametric circuit, can be calculated asfollows. The equivalent refractive index of the silica opticalwaveguides is n(λ_(p))=1.454 for pump light wavelength λ_(p)=775 nm andn(λ_(s))=1.444 for signal light wavelength λ_(s)=1550 nm. Thus,according to the equation (11),

ΔβL _(d)=(β(ω_(P))−β(ω_(S))−β(ω_(P)−ω_(S)))L_(d)=2π(n(λ_(p))/λ_(p)−2n(λ_(s))/λ_(s))L_(d)=2π(2.26/(775×10⁻⁹)−2×2.22/(1550×10⁻⁹))L _(d)=(2n−1)π (n is aninteger)   (21),

[0149] therefore, L_(d)=38.8 μm (n=1) is the condition for separatingthe signal light and the phase-conjugated light completely. Therefore,by setting the difference of the lengths of the silica opticalwaveguides 61, 62 and the difference of the lengths of the silicaoptical waveguides 63, 64 as an integral multiple of 38.8 μm, it becomespossible that the phase-conjugated light having the same wavelength asthat of the signal light is separated from the input signal lightcompletely and output from the output port 2.

[0150] Also in this embodiment, since the optical dispersive medium andthe optical nonlinear medium can be accumulated, it becomes possible toreduce cost by reducing number of optical circuit parts so that theoptical parametric circuit of the present invention is suitable for massproduction.

[0151] As quasi-phase matched optical waveguides which form the opticalnonlinear medium, materials of GaN family semiconductor, ZnSSe familysemiconductor, LiNbO₃, LiTaO₃, KTP(KLiOPO₄), KDP(KH₂PO₄), KNbO₃ and thelike can be used in addition to the GaAs/AlGaAs semiconductor. Inaddition, as substrates materials, Si, Ge, GaN and ZnSe semiconductorscan be used.

[0152] (Ninth Embodiment)

[0153] Each of embodiments from second to eighth includes the powersource 39(47), the photoreceiver 40 monitoring the pump light power andthe control circuit 41 such that the power source 39(47) obtainsfeedback from the control circuit 41 for compensating for the opticalpath-length error of the two optical paths of the nonlinear Mach-Zehnderinterferometer 24. Accordingly, the phase-conjugated light(wavelength-converted light) can be output from the output port 2. Onthe other hand, the ninth embodiment does not need such compensation forthe optical path-length error of the nonlinear Mach-Zehnderinterferometer by using a nonlinear loop mirror (nonlinear Sagnacinterferometer).

[0154]FIG. 17 shows the ninth embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1550 nm, the wavelength of the pump light is 775nm which corresponds to second-order harmonic wave of the signal light,and phase-conjugated light which has the same wavelength 1550 nm as thatof the input signal light is output.

[0155] The basic configuration of this embodiment is almost the same asthat of the seventh embodiment except that, in this embodiment, thenonlinear loop mirror 54 is used instead of the nonlinear Mach-Zehnderinterferometer 24. The nonlinear loop mirror 54 is configured such thattwo ports of the optical coupler 25 are connected via the phaseunmatched LiNbO₃ waveguide 51 used as the optical dispersive medium andthe quasi-phase matched LiNbO₃ waveguide 48 used as the second-orderoptical nonlinear medium like a loop. That is, in the right-handedoptical path, the phase mismatched LiNbO₃ waveguide 51 and thequasi-phase matched LiNbO₃ waveguide 48 are connected in this order. Inthe left-handed optical path, the quasi-phase matched LiNbO₃ waveguide48 and the phase mismatched LiNbO₃ waveguide 51 are connected in thisorder. Each of the right-handed optical path and the left-handed opticalpath corresponds to one of the two optical paths in the nonlinearMach-Zehnder interferometer. However, in this embodiment, since light inthe right-handed direction and light in the left-handed direction travelin the same optical path, the optical path length error which occursbetween the two optical path in the nonlinear Mach-Zehnderinterferometer does not occur. Thus, compensation for the error is notnecessary in this embodiment.

[0156] The wavelength conversion efficiency η_(PD) by using thequasi-phase matched LiNbO₃ waveguide 48 as the second-order opticalnonlinear medium becomes about 1.66(2.2 dB) under the same condition asthe fourth embodiment. In this case, amplification of thephase-conjugated light is also possible.

[0157] The length L_(d) of the phase mismatched LiNbO₃ waveguide 51 usedas the optical dispersive medium becomes L_(d)=9.7 μm under the samecondition as the sixth embodiment. That is, by configuring the opticalparametric circuit by using the phase mismatched LiNbO₃ waveguide of 9.7μm long as the optical dispersive medium, it becomes possible that thephase-conjugated light which has the same wavelength as the signal lightcan be separated from the signal light completely and be output from theoutput port 2.

[0158] In addition, in the above-mentioned configuration, when thesignal light and the pump light are applied to the two input ports ofthe optical coupler 25 respectively, the signal light is output from theoutput port 1, and the phase-conjugated light and the pump light areoutput from the output port 2. Also in this case, since the wavelengthdifference between the phase-conjugated light and the pump light islarge, these can be easily separated.

[0159] A configuration using the nonlinear loop mirror 54 instead of thenonlinear Mach-Zehnder interferometer 24 can be adopted not only for theseventh embodiment but also other embodiments. In the configurationusing the nonlinear loop mirror 54, since input and output ports for thesignal light and the phase-conjugated light become the same, an opticalisolator or an optical circulator is used (which is not shown in FIG.17).

[0160] (Tenth Embodiment)

[0161]FIGS. 18A and 18B show a tenth embodiment of the opticalparametric circuit of the present invention. In this embodiment, aconfiguration of the optical parametric circuit will be described inwhich the wavelength of the signal light is 1550 nm, the wavelength ofthe pump light is 775 nm which corresponds to second-order harmonic waveof the signal light, and phase-conjugated light which has the samewavelength 1550 nm as that of the input signal light is output.

[0162] In the embodiment shown in FIG. 18A, two sets of the nonlinearMach-Zehnder interferometers 24 including the optical path lengthcontrol system of the seventh embodiment are arranged betweenpolarization multiplexers 55 and 56 formed on the LiNbO₃ substrate 53.The polarization multiplexer 55 separates the signal light and the pumplight into p polarization component and s polarization component whichare applied to nonlinear Mach-Zehnder interferometers 24-1, 24-2respectively. Then, phase-conjugated lights of the p polarizationcomponent and the s polarization component which are output from thenonlinear Mach-Zehnder interferometers 24-1, 24-2 are synthesized in thepolarization multiplexer 56 and output. According to this structure, anoptical parametric circuit of a polarization-independent type whichoutputs constant phase-conjugated light irrespective of polarizationstate of input signal light can be realized.

[0163] The wavelength conversion efficiency η_(PD) in the second-orderoptical nonlinear medium and the length L_(d) of the optical dispersivemedium are the same as those of the seventh embodiment. In addition,instead of the nonlinear Mach-Zehnder interferometers 24-1 and 24-2, thenonlinear loop mirror 54 of the ninth embodiment can be used as shown inFIG. 18B.

[0164] As optical waveguides used for this embodiment, materials ofLiTaO₃, KTP(KLiOPO₄), KDP(KH₂PO₄), KNbO₃ and the like can be used inaddition to the LiNbO₃.

[0165] (Eleventh Embodiment)

[0166]FIG. 19 shows an eleventh embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1545 nm, the wavelength of the pump light is 1550nm, and wavelength-converted light of 1555 nm is output.

[0167] The basic configuration of this embodiment is almost the same asthat of the seventh embodiment except that, in this embodiment, the pumplight wavelength is in the wavelength range of the signal light, andthat the pump light of 1550 nm is converted into 775 nm (opticalcascading) in the second-order optical nonlinear medium, after that, thewavelength-converted light is generated by the same process as theseventh embodiment. In the seventh embodiment, since difference of thewavelengths between the signal light and the pump light is large, it isdifficult to couple both lights to fundamental waveguide mode of theoptical parametric circuit simultaneously. On the other hand, accordingto the present embodiment in which the optical cascading is used, sincethe wavelengths of the signal light and the pump light are in the samerange, it becomes easy to couple both lights to fundamental waveguidemode of the optical parametric circuit.

[0168] In this embodiment, since the pump light of the wavelength 1550nm is converted into the wavelength 775 nm by SHG process, conversionefficiency for the pump light power decreases. In addition, aquasi-phase matched LiNbO₃ waveguide 57 used as the second-order opticalnonlinear medium for the optical cascading is provided at the front ofthe optical coupler 25 of the nonlinear Mach-Zehnder interferometer 24.That is, a mixed light of the signal light and the pump light is appliedto the quasi-phase matched LiNbO₃ waveguide 57, and the signal light of1545 nm and the pump light in which the wavelength is converted to 775nm are applied to the optical coupler 25.

[0169] Here, the pump light of the wavelength 1550 nm and pump lightpower P_(P)=1000 mW is converted into the pump light of the wavelength775 nm and pump light power 800 mW by the quasi-phase matched LiNbO₃waveguide 57 of L_(n)=5 cm long. The pump light and the signal light aredivided into two by the optical coupler 25. In one optical path, thedivided light is applied to the quasi-phase matched LiNbO₃ waveguide 48via the phase mismatched LiNbO₃ waveguide 51 so that thewavelength-converted light is generated. In the other optical path, thepump light and the signal light are applied to the quasi-phase matchedLiNbO₃ waveguide 49 so that the wavelength-converted light is generated,in addition, the lights are applied to the phase mismatched LiNbO₃waveguide 52. Then, the signal light and the pump light, and thewavelength-converted light are separated by the optical coupler 26 andoutput from different output ports.

[0170] Assuming that the length of the quasi-phase matched LiNbO₃waveguides 48, 49 is L_(n)=6 cm, the effective cross-section area isA=50 μm², and the second-order nonlinear-dielectric constant isd=5×10⁻²³(MKS), the conversion parameter η_(PD)′ becomes9.2×10⁻⁵(mW⁻¹cm⁻²) according to the equation (18). When the power of thepump light of 775 nm is P_(P)=800 mW, the wavelength conversionefficiency η_(PD) becomes about 0.33(−4.8 dB).

[0171] The length L_(d) of the phase mismatched LiNbO₃ waveguides 51, 52used as the optical dispersive medium becomes L_(d)=9.7 μm under thesame condition as the sixth embodiment. That is, by configuring theoptical parametric circuit by using the phase mismatched LiNbO₃waveguide of 9.7 μm long as the optical dispersive medium, it becomespossible that the phase-conjugated light which has the same wavelengthas the signal light can be separated from the signal light completelyand be output from the output port 2.

[0172] The configuration using the optical cascading according to thisembodiment can be applied to the polarization-independent type opticalparametric circuit using the two nonlinear Mach-Zehnder interferometersshown in the tenth embodiment. This holds true for followingembodiments.

[0173] As optical waveguides used for this embodiment, materials ofLiTaO₃, KTP(KLiOPO₄), KDP(KH₂PO₄), KNbO₃ and the like can be used inaddition to the LiNbO₃.

[0174] (Twelfth Embodiment)

[0175]FIG. 20 shows a twelfth embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1545 nm, the wavelength of the pump light is 1550nm, and wavelength-converted light of 1555 nm is output.

[0176] The basic configuration of this embodiment is almost the same asthat of the eleventh embodiment except that, in this embodiment, thequasi-phase matched LiNbO₃ waveguides 57, 58 used as the second-orderoptical nonlinear medium for optical cascading are provided at the frontof two optical paths of the nonlinear Mach-Zehnder interferometer 24.

[0177] That is, the quasi-phase matched LiNbO₃ waveguide 57 is placedbetween the optical coupler 25 and the phase mismatched LiNbO₃ waveguide51 used as the optical dispersive medium and the quasi-phase matchedLiNbO₃ waveguide 58 is placed between the optical coupler 25 and thequasi-phase matched LiNbO₃ waveguide 49 used as the second-order opticalnonlinear medium. The quasi-phase matched LiNbO₃ waveguides 49, 58 formone waveguide substantially.

[0178] Here, the pump light of the wavelength 1550 nm and pump lightpower P_(P)=1000 mW is divided into two optical paths by the opticalcoupler 25 and is converted into the pump light of the wavelength 775 nmand total power 800 mW by the quasi-phase matched LiNbO₃ waveguides 57,58 of L_(n)=5 cm long. In one optical path, the pump light and thesignal light are applied to the phase mismatched LiNbO₃ waveguide 51 andare applied to the quasi-phase matched LiNbO₃ waveguide 48 so that thewavelength-converted light is generated. In the other optical path, thepump light and the signal light are applied to the quasi-phase matchedLiNbO₃ waveguide 49 so that the wavelength-converted light is generated,in addition, the lights are applied to the phase mismatched LiNbO₃waveguide 52. Then, the signal light and the pump light, and thewavelength-converted light are separated by the optical coupler 26 andoutput from different output ports.

[0179] Here, the wavelength conversion efficiency η_(PD) for thequasi-phase matched LiNbO₃ waveguides 48, 49 used as the opticalnonlinear medium, and the length L_(d) of the phase mismatched LiNbO₃waveguides 51, 52 used as the optical dispersive medium are the same asthose of the eleventh embodiment.

[0180] (Modification Of The Twelfth Embodiment)

[0181]FIG. 21 shows a configuration example 1 of the optical dispersivemedium and the second-order optical nonlinear medium in the twelfthembodiment. FIGS. 22A and 22B show configuration examples 2 of theoptical dispersive medium and the second-order optical nonlinear mediumin the twelfth embodiment.

[0182] As for configurations shown in the figures, in the quasi-phasematched LiNbO₃ waveguides 57, 58 used as the second-order opticalnonlinear medium for optical cascading and the quasi-phase matchedLiNbO₃ waveguides 48, 49 used as the second-order optical nonlinearmedium for generating the wavelength-converted light, absolute values ofnonlinear-dielectric constant d (used in the equation (4)) for thedipole-inversion regions 50 and the dipole-noninversion regions 70 arethe same and the signs of the values are opposite. That is, the lengthsof the dipole-inversion region 50 and the dipole-noninversion region 70are the same wherein the length is one-half of dipole-inversion cycle.

[0183] As shown in FIGS. 21 and 22A, in the quasi-phase matched LiNbO₃waveguides 48, 49, the dipole-inversion regions 50 and thedipole-noninversion regions 70 are formed alternately in which thelength of each region is 9.7 μm. Accordingly, the firstdipole-noninversion region 70 in the quasi-phase matched LiNbO₃waveguide 48 can be used as the phase mismatched LiNbO₃ waveguide 51 as9.7 μm long optical dispersive medium, and the last dipole-noninversionregion 70 in the quasi-phase matched LiNbO₃ waveguide 49 can be used asthe phase mismatched LiNbO₃ waveguide 52 as another 9.7 μm long opticaldispersive medium. That is, from outward appearances, there is nodifference between the optical dispersive medium and the opticalnonlinear medium.

[0184] While the FIG. 21 shows a configuration in which the positions ofthe dipole-inversion regions 50 and the dipole-noninversion regions 70between the quasi-phase matched LiNbO₃ waveguides 57 and 58 become thesame in light traveling direction, as shown in FIG. 22A, thedipole-inversion regions 50 and the dipole-noninversion regions 70 canbe placed alternately also in the quasi-phase matched LiNbO₃ waveguides57 and 58.

[0185] In addition, as a modification of the configuration shown in theFIG. 22A, the first dipole-noninversion region 70 of the quasi-phasematched LiNbO₃ waveguide 57 can be used as the phase mismatched LiNbO₃waveguide 51 used as one optical dispersive medium as shown in FIG. 22B.

[0186] (Thirteenth Embodiment)

[0187]FIG. 23 shows a thirteenth embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1545 nm, the wavelength of the pump light is 1550nm, and wavelength-converted light of 1555 nm is output.

[0188] The basic configuration of this embodiment is almost the same asthat of the ninth embodiment except that, in this embodiment, the pumplight wavelength is in the wavelength range of the signal light, andthat the pump light of 1550 nm is converted into 775 nm (opticalcascading) in the second-order optical nonlinear medium, after that, thewavelength-converted light is generated by the same process as the ninthembodiment. That is, the quasi-phase matched LiNbO₃ waveguide 57 used asthe second-order optical nonlinear medium for optical cascading isprovided at the front of the optical coupler 25 of the nonlinear loopmirror 54. A mixed light of the signal light and the pump light isapplied to the quasi-phase matched LiNbO₃ waveguide 57, and the signallight of 1545 nm and the pump light in which the wavelength is convertedto 775 nm are applied to the optical coupler 25.

[0189] The wavelength conversion efficiency η_(PD) by using thequasi-phase matched LiNbO₃ waveguide 48 as the second-order opticalnonlinear medium becomes about 1.66(2.2 dB) under the same condition asthe fourth embodiment. In this case, amplification of thewavelength-converted light is also possible.

[0190] The length L_(d) of the phase mismatched LiNbO₃ waveguide 51 usedas the optical dispersive medium becomes L_(d)=9.7 μm under the samecondition as the sixth embodiment. That is, by configuring the opticalparametric circuit by using the phase mismatched LiNbO₃ waveguide of 9.7μm long as the optical dispersive medium, it becomes possible that thewavelength-converted light of 1555 nm can be separated from the signallight completely and be output from the output port 2.

[0191] (Fourteenth Embodiment)

[0192]FIG. 24 shows a fourteenth embodiment of the optical parametriccircuit of the present invention. In this embodiment, a configuration ofthe optical parametric circuit will be described in which the wavelengthof the signal light is 1549.5 nm, the wavelengths of the pump lights are1545 nm and 1555 nm, and wavelength-converted light of 1550.5 nm isoutput.

[0193] The basic configuration of this embodiment is almost the same asthat of the twelfth embodiment except that, in this embodiment, twowavelengths are used as the pump lights, and that the pump lights ofwavelengths 1545 nm and 1555 nm are coupled and converted into 775 nm(optical cascading) in the second-order optical nonlinear medium. Afterthat, the wavelength-converted light of 1550.5 nm can be generated bythe same process as the twelfth embodiment. When, as the pump lights,two waves in which the wavelength of the signal light exists in thecenter of the two waves are used, phase-conjugated light having the samewavelength as the signal light wavelength can be output.

[0194] As optical waveguides used for this embodiment, materials ofLiTaO₃, KTP(KLiOPO₄), KDP(KH₂PO₄), KNbO₃ and the like can be used inaddition to the LiNbO₃.

[0195] As mentioned above, according to the present invention, since theoptical parametric circuit uses the nonlinear Mach-Zehnderinterferometer or the nonlinear loop mirror in which the opticaldispersive medium and the second-order optical nonlinear medium arecombined, the signal light and the pump light, and thewavelength-converted light (or the phase-conjugated light) can beseparated and output irrespective of difference of wavelengths.Accordingly, the optical filter for cutting input signal light which isnecessary for conventional optical parametric wavelength conversionbecomes unnecessary. In addition, since guard band necessary forfiltering becomes also unnecessary, given wavelength space can be usedefficiently.

[0196] Further, when the phase-conjugated light is generated, thephase-conjugated light which has the same wavelength as that of theinput signal light can be generated and these lights can be completelyseparated. Thus, functions which were impossible to realize by theconventional technique can be realized. By applying the presentinvention to an optical transmission system, spreading of signal opticalspectrum due to fiber nonlinear effect, which is a problem in opticalfiber transmission, can be suppressed. Accordingly, transmissionpossible distance can be further increased and transmission quality canbe further improved.

[0197] In addition, by adjusting the pump light power, the length of theoptical nonlinear medium and the like properly, optical parametricamplification can be performed for the wavelength-converted light (orthe phase-conjugated light) with respect to the input signal light.

[0198] Further, since the optical parametric circuit of the presentinvention can convert wavelengths at high speed, it can be used as afunction device of an optical wavelength router.

[0199] The present invention is not limited to the specificallydisclosed embodiments, and variations and modifications may be madewithout departing from the scope of the invention.

What is claimed is:
 1. An optical parametric circuit comprising: a firstoptical path which connects an output port of a first optical couplerand an input port of a second optical coupler; a second optical pathwhich connects an output port of said first optical coupler and an inputport of said second optical coupler; an optical dispersive medium and asecond-order optical nonlinear medium provided in each of said firstoptical path and said second optical path; and wherein said opticaldispersive medium and said second-order optical nonlinear medium in saidfirst optical path are placed in the reverse order of said opticaldispersive medium and said second-order optical nonlinear medium whichare placed in said second optical path.
 2. An optical parametric circuitcomprising: a nonlinear Mach-Zehnder interferometer which includes afirst optical path which connects an output port of a first opticalcoupler and an input port of a second optical coupler; a second opticalpath which connects an output port of said first optical coupler and aninput port of said second optical coupler; a first optical dispersivemedium and a first second-order optical nonlinear medium provided insaid first optical path; a second optical dispersive medium and a secondsecond-order optical nonlinear medium provided in said second opticalpath; and wherein said first optical dispersive medium and said firstsecond-order optical nonlinear medium are placed in the reverse order ofsaid second optical dispersive medium and said second second-orderoptical nonlinear medium which are placed in said second optical path.3. The optical parametric circuit as claimed in claim 2 , wherein amixed light of a signal light and a pump light is applied to an inputport of said first optical coupler, said signal light and said pumplight are output from an output port of said second optical coupler, anda wavelength-converted light or a phase-conjugated light for said inputsignal light is output from another output port of said second opticalcoupler.
 4. The optical parametric circuit as claimed in claim 2 ,wherein a signal light and a pump light are applied to one input portand another input port of said first optical coupler respectively, saidsignal light is output from an output port of said second opticalcoupler, and a wavelength-converted light or a phase-conjugated lightfor said input signal light and said pump light are output from anotheroutput port of said second optical coupler.
 5. The optical parametriccircuit as claimed in claim 2 , further comprising: a control part,provided in at least one of said optical paths, which controls effectiveoptical path-length of said at least one of said optical paths; whereinsaid control part controls said effective optical path-length such thatinterference condition of two signal lights which pass through saidfirst and second second-order optical nonlinear mediums, and,interference condition of two wavelength-converted lights or twophase-conjugated lights which pass through said first and secondsecond-order optical nonlinear mediums are different by (2n−1) π,wherein n is an integer.
 6. An optical parametric circuit comprising: anoptical coupler; a nonlinear loop mirror; and wherein one output port ofsaid optical coupler is connected to another output port of said opticalcoupler via an optical dispersive medium and a second-order opticalnonlinear medium by said nonlinear loop mirror.
 7. The opticalparametric circuit as claimed in claim 6 , wherein a mixed light of asignal light and a pump light is applied to an input port of saidoptical coupler, and a wavelength-converted light or a phase-conjugatedlight for said input signal light is output from another input port ofsaid optical coupler.
 8. The optical parametric circuit as claimed inclaim 6 , wherein a signal light is input to and output from one inputport of said optical coupler, a pump light is input to and output fromanother input port of said optical coupler, and a wavelength-convertedlight or a phase-conjugated light for said input signal light is outputfrom said another input port of said optical coupler.
 9. The opticalparametric circuit as claimed in claim 2 , wherein each of said firstand second second-order optical nonlinear mediums is formed as anoptical waveguide.
 10. The optical parametric circuit as claimed inclaim 6 , wherein said second-order optical nonlinear medium is formedas an optical waveguide.
 11. The optical parametric circuit as claimedin claim 2 , wherein said optical dispersive mediums and saidsecond-order optical nonlinear mediums are formed as optical waveguidesformed on a substrate of a second-order optical nonlinear medium; andeach function of said optical dispersive mediums or said second-orderoptical nonlinear mediums is realized by structural parameters of saidoptical waveguides.
 12. The optical parametric circuit as claimed inclaim 6 , wherein said optical dispersive medium and said second-orderoptical nonlinear medium are formed as an optical waveguide formed on asubstrate of a second-order optical nonlinear medium; and each functionof said optical dispersive medium and said second-order opticalnonlinear medium is realized by structural parameters of said opticalwaveguide.
 13. The optical parametric circuit as claimed in claim 11 ,wherein said optical couplers are formed as said optical waveguidesformed on said substrate of said second-order optical nonlinear medium.14. The optical parametric circuit as claimed in claim 12 , wherein saidoptical coupler is formed as said optical waveguide formed on saidsubstrate of said second-order optical nonlinear medium.
 15. The opticalparametric circuit as claimed in claim 9 , wherein each of said firstand second second-order optical nonlinear mediums is quasi-phasematched.
 16. The optical parametric circuit as claimed in claim 10 ,wherein said second-order optical nonlinear medium is quasi-phasematched.
 17. An optical parametric circuit comprising: two opticalparametric circuit each of which optical parametric circuit comprising:a nonlinear Mach-Zehnder interferometer which includes a first opticalpath which connects an output port of a first optical coupler and aninput port of a second optical coupler; a second optical path whichconnects an output port of said first optical coupler and an input portof said second optical coupler; a first optical dispersive medium and afirst second-order optical nonlinear medium provided in said firstoptical path; a second optical dispersive medium and a secondsecond-order optical nonlinear medium provided in said second opticalpath; and wherein said first optical dispersive medium and said firstsecond-order optical nonlinear medium are placed in the reverse order ofsaid second optical dispersive medium and said second second-orderoptical nonlinear medium which are provided in said second optical path;a polarization separation part which separates polarizations of a signallight and a pump light, and applies polarization components to said twooptical parametric circuits respectively; and a polarizationmultiplexing part which polarization-multiplexes polarization componentsof a wavelength-converted light or a phase-conjugated light which areoutput from said two optical parametric circuits and outputs amultiplexed light.
 18. An optical parametric circuit comprising: twooptical parametric circuit each of which optical parametric circuitcomprising: an optical coupler; a nonlinear loop mirror; and wherein oneoutput port of said optical coupler is connected to another output portof said optical coupler via an optical dispersive medium and asecond-order optical nonlinear medium by said nonlinear loop mirror; apolarization separation part which separates polarizations of a signallight and a pump light, and applies polarization components to said twooptical parametric circuits respectively; and a polarizationmultiplexing part which polarization-multiplexes polarization componentsof a wavelength-converted light or a phase-conjugated light which areoutput from said two optical parametric circuits and outputs multiplexedlight.
 19. The optical parametric circuit as claimed in claim 2 ,wherein wavelength difference between a signal light and a pump lightwhich are to be applied to said nonlinear Mach-Zehnder interferometer iswithin 150 nm; and said optical parametric circuit further comprising anSHG part which generates second-order harmonic wave from said pump lightby SHG process and applies said second-order harmonic wave to saidnonlinear Mach-Zehnder interferometer.
 20. The optical parametriccircuit as claimed in claim 6 , wherein wavelength difference between asignal light and a pump light which are to be applied to said nonlinearloop mirror is within 150 nm; and said optical parametric circuitfurther comprising an SHG part which generates second-order harmonicwave from said pump light by SHG process and applies said second-orderharmonic wave to said nonlinear Mach-Zehnder interferometer.
 21. Theoptical parametric circuit as claimed in claim 19 , wherein said SHGpart are inserted into each of said two optical paths in said nonlinearMach-Zehnder interferometer.
 22. The optical parametric circuit asclaimed in claim 19 , wherein two pump lights in which wavelengthdifference between said signal light and each of said two pump lights iswithin 150 nm are used.
 23. The optical parametric circuit as claimed inclaim 20 , wherein two pump lights in which wavelength differencebetween said signal light and each of said two pump lights is within 150nm are used.
 24. The optical parametric circuit as claimed in claim 2 ,further comprising a pump light source which generates a pump light tobe applied to said nonlinear Mach-Zehnder interferometer.
 25. Theoptical parametric circuit as claimed in claim 6 , further comprising apump light source which generates a pump light to be applied to saidnonlinear loop mirror.